Catechol O-methyltransferase. 6. Affinity labeling with N-haloacetyl-3,5-dimethoxy-4-hydroxyphenylalkylamines

Studies on an affinity label for the sulfuryl acceptor binding site in an aryl sulfotransferase

Active site-directed affinity labeling was utilized to elucidate peptide sequences at the binding site for sulfuryl acceptors in rat hepatic aryl sulfotransferase (AST) IV (also known as tyrosine-ester sulfotransferase, EC 2.8.2.9). The affinity labeling reagent, N-bromoacetyl-4-hydroxyphenylamine, was designed on the basis of substrate specificity studies with para-substituted phenols, utilization of a bromoacetamido group for reactivity with active site amino acid residues and its similarity to acetaminophen, a known substrate for aryl (phenol) sulfotransferases. AST IV utilized N-bromoacetyl-4-hydroxyphenylamine as a substrate with kinetic constants that compared favorably to those obtained with acetaminophen. Incubation of AST IV with N-bromoacetyl-4-hydroxyphenylamine at pH 7.0 in the absence of PAPS and other substrates resulted in an irreversible inactivation of the enzyme that was both time- and concentration-dependent. [14C]-N-bromoacetyl-4-hydroxyphenylamine was synthesized and used to analyze the regions of protein sequence that were involved in the binding of the affinity label. AST IV was incubated with [14C]-N-bromoacetyl-4-hydroxyphenylamine, hydrolyzed with endoproteinase Lys-C and the labeled peptides were purified by HPLC. Control incubations of AST IV with the affinity label in the presence of 4-propylphenol and PAP were utilized to ascertain the specificity of the interaction. Sequence analysis of the labeled peptides, carried out by automated Edman degradation, revealed labeling sites on cysteine (Cys-232, Cys-283 and Cys-289) and lysine (Lys-286) residues near the C-terminus of the protein. The locations of these labeling sites were further evaluated both by sequence-alignment with other sulfotransferases and by theoretical calculations on predicted secondary structure.

Inhibition of catechol-O-methyltransferase by 1-vinyl derivatives of nitrocatechols and nitroguaiacols. Kinetics of the irreversible inhibition by 3-(3-hydroxy-4-methoxy-5-nitro benzylidene)-2,4-pentanedione

It is well known that activated alkene derivatives react with thiol groups according to a Michael's addition reaction. On the basis of the presence of at least one thiol group essential for the activity of catechol-O-methyltransferase (COMT), several 1-vinyl derivatives of nitrocatechol and nitroguaiacol were synthesized and tested as potential irreversible active site-directed inhibitors of COMT. All the synthesized products were potent inhibitors of partially purified pig liver COMT. However, the inhibition was reversible in most cases, with the exception of 3-(3-hydroxy-4-methoxy-5-nitrobenzylidene)-2,4-pentanedione (compound 2) which inhibited COMT in an irreversible manner. When the inhibition of COMT was measured as a function of the length of time of pre-incubation with 2, biphasic kinetics were observed, suggesting the modification of at least two thiol groups which are essential for COMT activity. The analysis of the two parts of the inhibition curve as a function of the inhibitor concentration showed that compound 2 modified the more reactive group in a non-specific manner, while it behaved as an active site-directed inhibitor on the second slow-reacting thiol group. Importantly, a saturating concentration of S-adenosyl-L-methionine (AdoMet) in the pre-incubation mixture gave pseudo-first order kinetics, suggesting total protection of one thiol group. Magnesium ions had no effect on the protection of COMT by AdoMet. In the presence of 3,5-dinitrocatechol (DNC) slight protection of COMT was observed; when the inactivation of both groups was analysed independently, protection of the specifically modified group was detected, while the reaction with the other group was faster in the presence of DNC. When both AdoMet and DNC were present, inactivation of COMT by 2 was not observed, suggesting that both reacting groups are located at or near the active site.

Kinetic and inhibition studies on catechol-O-methyltransferase affinity labelling by N-(3,4-dihydroxyphenyl)maleimide

Initial velocity and product inhibition studies have been performed on soluble catechol-O-methyltransferase which has been partially purified from pig liver. The results are consistent with an ordered reaction mechanism, in which S-adenosyl-L-methionine (AdoMet) is the leading substrate. The enzyme is irreversibly inhibited by maleimide derivatives in a biphasic manner, which suggests a differential reaction with two thiol groups. N-(3,4-Dihydroxyphenyl)maleimide, which has a reactive moiety (maleimide ring) and an affinity moiety (catechol ring), acts as an affinity labelling compound on the more reactive SH group; AdoMet and Mg2+ protect against this modification. Total protection of this SH group results in a pseudo-first-order inhibition of the enzyme, with the apparent rate constant being proportional to the inhibitor concentration. All the other maleimide derivatives studied inhibited the enzyme by reacting with one of the two SH groups in a non-specific manner. The reaction of the other, more reactive, SH group was either specific (active-site-directed) or non-specific, depending on the substituent present in the affinity moiety and also on the length of an intermediate chain of methylene groups present between this moiety and the reactive maleimide ring. In the presence of both AdoMet and Mg2+, 3,5-dinitrocatechol, a reversible inhibitor of the enzyme which is competitive with respect to the catechol substrate, protects the enzyme from inactivation by any of the maleimide derivatives. The adducts of these maleimide derivatives formed with dithiothreitol inhibit the enzyme reversibly, showing inhibition patterns that are consistent with the mechanism deduced from the initial velocity and product inhibition studies.

Inhibition of catechol-O-methyltransferase by N-(3,4-dihydroxyphenyl) maleimide

Catechol-O-methyltransferase (COMT) is inhibited rapidly and irreversibly by N-(3,4-dihydroxyphenyl) maleimide. S-adenosylmethionine (AdoMet) and magnesium ions protect the enzyme from inactivation by this compound, but no protection is observed by the catechol substrate. However, the corresponding succinimide analogue shows a reversible inhibition of COMT, which is competitive with pyrocatecholphthalein and non-competitive with AdoMet. Amino-group reagents also inhibit COMT and this inhibition is protected by AdoMet, suggesting that sulphydryl and amino groups essential for activity are located in an AdoMet-binding site on COMT. The maleimide derivative may be considered to be an active-site directed inhibitor.

Evidence for sulfhydryl groups at the active site of catechol-O-methyltransferase

Earlier studies using affinity labeling reagents have suggested the existence of two nucleophilic groups at the active site of catechol-O-methyltransferase (S-adenosyl-L-methionine:catechol O-methyltransferase, EC 2.1.1.6). Both nucleophilic residues are critical for catalytic activity. In an effort to elucidate the nature of these residues and to further characterize the relationship between the chemical structure and the catalytic function of this enzyme, inactivation studies using N-ethylmaleimide were undertaken. Inactivation of the enzyme by N-ethylmaleimide under pseudo first-order conditions exhibited a non-linear relationship between the log of the fraction of enzyme activity remaining and preincubation time. Kinetic analysis of this inactivation process suggested the modification by N-ethylmaleimide of two residues at the active site of the enzyme, both crucial for catalytic activity. Detailed analysis of the inactivation process including substrate protection studies, pH profiles of inactivation, and incorporation studies using N-ethyl[2,3-14C2]maleimide provided additional evidence to support this conclusion.